The changes in growth hormone levels in the pituitary gland and in the blood plasma of rats under various conditions are being studied by means of immunochemical techniques. Conditions to be studied include hormone levels after birth up through puberty to senility. The influence of various types of stress will also be studied. In the second general area of investigation the relationship between man and other primates as well as those between higher and lower vertebrates are being studied by means of immunologic procedures employing antisera to pituitary growth hormone and prolactin. The findings from this investigation will be compared to the results obtained with nonimmunologic procedures. An attempt is being made in these studies, therefore, to correlate evolutionary trends with modifications in hormone structure determined by immunochemical procedures.